Injection molding is a well-known method for forming plastic articles (which will be generically referred to herein as “parts”). The injection molding process can usually be regarded as including the following steps:
(1) A clamping step, wherein portions of a mold are brought together to define a mold cavity into which molten plastic is to be injected.
(2) A filling or injection step, wherein plastic is injected into the mold via a sprue (the entry to the mold), runners (one or more passages branching out from the sprue), and gates (one or more openings from which the runners open onto the mold cavity). Injection can be achieved in a number of different ways depending on the configuration of the injection molding apparatus. A common arrangement, which will be assumed as an example throughout the remainder of this document, is to have a screw which is rotatable within a barrel (which is generally provided with heaters about the barrel circumference), and which is also translatable within the barrel by a hydraulic ram. Plastic feedstock is fed into the barrel (generally by a hopper) and sheared, mixed and melted by the action of the rotating screw and the heaters. The screw and ram retreat within the barrel as the screw's rotation urges molten plastic to the front of the barrel, and then injection is effected by pushing the screw forward on the ram, injecting the molten plastic into the sprue at the front of the barrel. The ram (and screw) movement is usually controlled to attain a desired velocity profile—usually constant velocity—in an attempt to achieve desired flow of molten plastic within the cavity (e.g., a uniform melt front). Once the mold cavity is filled (or is believed to be filled), the filling/injection step is completed, with the time of completion often being referred to as the “switchover point” or “switchover point.” Since the switchover point, i.e., the nominal time of complete cavity filling, often cannot be determined precisely (at least without expensive sensors and/or other equipment modifications), it is usually set with reference to a closely related process or machine parameter—for example, it may be set at some preset distance by which the ram and screw have advanced (or some preset time after the ram and screw have advanced), at some pressure measured by a sensor within the mold cavity or injection barrel, on the ram, etc.
(3) A packing or holding step, wherein a small amount of additional plastic is packed to compensate for shrinkage (e.g., by urging the screw forward in the barrel by an additional small amount). Here, the ram (and thus the screw) are usually moved forward via pressure control, i.e., to attain some desired packing or holding pressure within the mold cavity, rather than via velocity control.
(4) A cooling and recovery step, wherein the plastic within the mold is allowed to at least partially cool and solidify, and the screw injector begins recharging with additional plastic material. Some cooling may also be regarded as occurring during the packing/holding step, though the cooling/recovery step may be regarded as distinct from the holding/packing step, and as occurring once the plastic at the gate(s) has solidified (i.e., as occurring once no more plastic can be admitted to the mold cavity).
(5) A mold opening step, wherein the mold opens.
(6) An ejection step, wherein the molded part is ejected from the mold cavity, often by one or more pins in the mold cavity walls being actuated to push the part from the mold cavity.
The injection molding cycle can then repeat, starting over at step (1) above. Note that the various steps above can be combined, subdivided, or otherwise altered or recharacterized; see, e.g., the discussion provided in U.S. Pat. No. 7,037,452.
Naturally, it is highly desirable for the injection molding process to result in parts which are uniform from cycle to cycle, and which have high dimensional accuracy and otherwise suitable qualities. Such qualities have previously been sought by implementing control schemes which attempt to control machine and/or process parameters (e.g., mold pressure, temperature, etc.) so that subsequent injection cycles are uniform, with the assumption being that if the machine operates in the same manner with each cycle, uniform quality will result. However, even where plastic feedstock is uniform (and thus factors such as varying feedstock density do not play a role), uniform quality has proven to be a difficult goal to attain owing to variations in machine, process, and material (plastic) parameters from cycle to cycle.
One factor that has been found to be a useful indicator of molded part quality is part weight. Research has found that variations in part weight are highly correlated with variations in part dimensions (which are usually undesirable, assuming part uniformity is desired). Thus, part weight often serves as an effective indicator of part dimensional uniformity, and thus part quality. Prior injection molding control schemes have been implemented wherein machine and process parameters are adapted after each cycle in response to the measured part weight, and such schemes are useful in eliminating long-term quality discrepancies (in that part nonuniformities are usually reduced or eliminated within a few cycles). However, short-term discrepancies still exist: some parts will vary from their desired quality targets, and while these generate error signals which are then used to adjust parameters and correct for the discrepancies in subsequent cycles, it would nonetheless be beneficial to further eliminate discrepancies and enhance part quality.